Electrical discharge machines serve for highly precise machining of electrically conductive workpieces. Therein material is removed by means of an electrode and a spark discharge.
Wire electrodes are used, for instance, to make cylindrical or conical cuts and to produce stamps or matrices in this manner. A section of a wire running between two guide heads is commonly referred to as a wire electrode. A tension generated in the wire path system ensures in this case the necessary straight linear shape of the electrode.
The term "guide head" is understood to mean a unit which is moved as a whole for the spatial positioning of the electrode. Generally, the actual wire guide is ordinarily formed in the interior of such a unit.
Each guide head is, for instance, seated in a cross carriage. Consequently, each can be displaced independently of the other in one of two typically parallel X-Y planes. It is practical in this regard if at least one of the cross carriages can be moved perpendicular to these planes in the Z direction. In this way it is possible to adjust the wire electrode to any given spatial position.
Cylindrical cuts are obtained in an EDM machine constructed in this manner by displacing both guide heads synchronously on similar paths in their respective guide planes. Conical cuts, on the contrary, are obtained by an asynchronous and/or unequal displacement of the guide heads.
For cylindrical cuts, therefore, the guide heads are generally exactly superimposed in the Z direction. For conical cuts, on the other hand, they are offset relative to one another.
For all cuts, what matters is merely the relative motion between the wire electrode and the workpiece. Cylindrical cuts are also obtained, for instance, if the workpiece is displaced in an X-Y direction with respect to a stationary wire electrode parallel to the Z axis, and conical cuts can be obtained if the workpiece and one guide head are moved, while the other guide head is held in place. It is also possible to pivot both guide heads each about an axis transverse to the Z axis and also to rotate them about the Z axis (in this case, each wire guide describes a circular orbit about the Z axis).
It is immaterial to the present invention whether the EDM machine is constructed in such a manner or a similar one. The only essential point is that in all these embodiments the guide heads are mutually offset or at least point in different directions. For reasons of simplicity, therefore, reference is made exclusively to the variant with two cross carriages in the description.
In case of a conical cut, the outlets of the guide heads for the wire electrode commonly point in or against the Z direction, while the wire electrode itself runs at an angle to it.
On the one hand, the erosion wire has a certain stiffness. Consequently, it initially leaves the wire outlet in or against the Z direction. Only as it continues is it bent towards the respective opposing guide head. The wire electrode thus no longer runs straight, but slightly curved (this is illustrated in FIG. 10).
On the other hand, the flushing of the wire electrode is disturbed. In the ideal case, the flushing should be done by a coaxial jet which runs symmetrically to the wire electrode. However, in the situation described above, this is no longer possible. This causes a deviation of the wire electrode out of its path by an asymmetrical flushing jet. The cooling action of the flushing jet is also no longer optimal.
German Patent No. DE 3041613 C1 (AGIE) attempts to remedy this problem by a cardan suspension of the guide heads. In this way it is possible to pivot the guide heads in the direction of the wire electrode. Such a design is relatively elaborate, however, and requires a great deal of space. It is also difficult to realize the design sufficiently stiffly. Consequently, it is very expensive to meet the current precision standards for erosion machines with this teaching.
German Patent No. DE 3201544 A1 (EROWA) takes a similar path by seating the guide heads so as to pivot and coupling them via a coupling transmission. In this way the control effort is reduced, which is nearly inconsequential, however, given the present state of development of control electronics. Moreover, such a construction increases the disadvantages cited with regard to the previous publication. This applies in particular to the stiffness problems.